Modulation systems and methods are widely used in transmitters to modulate information including voice and/or data onto a carrier. The carrier may be a final carrier or an intermediate carrier. The carrier frequency can be in UHF, VHF, RF, microwave or any other frequency band. Modulators are also referred to as "mixers" or "multipliers". For example, in a mobile radiotelephone, a modulator is used for the radiotelephone transmitter.
FIG. 1 illustrates a conventional IQ modulator. As shown in FIG. 1, an IQ modulator 10, also referred to as a "quadraphase modulator" or a "quadrature modulator" includes a quadrature splitter 20, also known as a 90.degree. phase shifter, and a pair of multipliers 16a, 16b coupled to the quadrature splitter. A controlled oscillator 15, such as a Voltage Controlled Oscillator (VCO), is coupled to the quadrature splitter 20 to produce 90.degree. phased shifted oscillator signals. In-phase (I) data 11a and quadrature-phase (Q) data 11b are coupled to a respective multiplier or mixer 16a, 16b. Digital input data is converted to analog data by I Digital-to-Analog Converter (DAC) 14a and Q DAC 14b, respectively. The outputs of the respective DACs 14a and 14b are applied to the respective low pass filters 12a and 12b to provide the respective I and Q data inputs 11a and 11b. The modulator modulates the input data on a carrier 13, by summing the outputs of the multipliers 16a, 16b at a summing node 218. The modulated carrier is amplified by a power amplifier 22 and transmitted via an antenna 24.
In modern radiotelephone communications, mobile radiotelephones continue to decrease in size, cost and power consumption. In order to satisfy these objectives, it is generally desirable to provide IQ modulation systems and methods that can provide high power modulation while reducing the amount of battery power that is consumed. Unfortunately, the power amplifier 22 of an IQ modulator may consume excessive power due to efficiency limitations therein. More specifically, it is known to provide a linear class-A or class-AB power amplifier 22 that may have efficiencies as low as 30 percent or less. Thus, large amounts of battery power may be wasted as heat. Moreover, the noise figure of a conventional IQ modulator may be excessive so that high cost Surface Acoustic Wave (SAW) filters may need to be used.
It is also known to separately modulate the amplitude and phase of an input signal using an "rTheta" technique. In the rTheta technique, the phase is modulated at the oscillator, and the amplitude is modulated at the power amplifier stage. Unfortunately, the rTheta technique may require the oscillator phase lock loop to support the phase modulation bandwidth. With wide bandwidth radiotelephone signals such as TDMA and CDMA signals, it may be increasingly difficult to provide the requisite bandwidth in the oscillator phase lock loop.